Touch Switch Circuit With MOSFET

Created by: Jonsen Li

Overview:

The simple touch switch LED circuit utilizes biasing characteristics of the MOSFET.

MOSFET stands for Metal-oxide-semiconductor field effect transistors. It is a voltage controlled device meaning that the current passing through the device is controlled by the voltage between two terminals.

Parts you will need:

A power MOSFET (IRFZ-44 NPN) (Lee's ID: 71211)

9V battery (Lee's ID: 83741)

12V LED bulb (Lee's ID: 5504)

Jumper wires (Lee's ID: 21802)

Breadboard (Lee's ID: 10686)

9V battery clip (Lee's ID: 653)

Since the mosfet is a voltage controlled device, it is very sensitive to electrostatic discharges and may be damaged due to the static charges flowing through the terminals.

Simply connect the jumper terminals to the legs of the MOSFET

For IRFZ-44:

The left leg is the gate terminal (white jumper)

The middle is the drain terminal (brown jumper)

The right leg is the source terminal (grey jumper)

To turn the LED on, simply touch the drain terminal and gate terminal at the SAME time.

To turn the LED off, touch the source terminal and gate terminal at the SAME time

The logic behind this project is the characteristics of the MOSFET:

For the LED to light up, the MOSFET must fully ON, which means Vds > Vgs – Vt. Since MOSFETs are voltage-controlled transistors, touching the drain and gate terminal at the same time will “short” them, therefore allowing the MOSFET to be fully ON.

On the other hand, touching the gate and source terminal will turn the MOSFET fully OFF, because it cannot satisfy the overdrive voltage (Vov) requirement (Vov = Vgs – Vt, Vgs = 0V).

If you have difficulties turning the circuit on and off, wetting your hands may help.

Here is a quick video demonstration of the touch switch in action.